1. Field of the Invention
This invention relates generally to a video tape recorder (VTR) of the rotary head type and, more particularly is directed to a rotary head VTR in which a pulse code modulated (PCM) signal and a video signal are recorded in first and second areas, respectively, of an oblique or slant track on a magnetic tape and a subcode signal is recorded in an area of the track between the first and second areas.
2. Description of the Prior Art
Referring initially to FIG. 1A, it will be seen that a small rotary head VTR, for example, an 8 mm VTR, records signals on a magnetic tape 1 in successive slant tracks extending obliquely across the tape, and one of which is indicated at 31. In the track format used in the 8 mm VTR, each of the slant tracks 31 has a length corresponding to the rotational movement of a respective one of two diametrically opposed rotary heads 2a and 2b (FIG. 4) through a rotational angle of 221.degree.. A track portion 31S corresponding to rotation of one of the rotary heads 2a and 2b through a rotational angle of 36.degree. from the starting end of the track 31, that is, the end at which the rotary head enters into contact with the tape 1, is used for the recording of an audio signal. A track portion 31V corresponding to rotation of the head 2a or 2b through the remaining angle of 185.degree. is used for the recording of the video signal.
As shown particularly on FIG. 1B, the track portion 31S is divided into successive areas 41-45 having respective lengths a-e. More specifically, the area 41 is a scan start area at which the rotary head 2a or 2b enters into contact with the tape 1, and which, for example, has a length corresponding to rotation of the respective head through an angle of 2.04.degree.. The next area 42 is a preamble area having a length b which, for example, is equivalent to the rotation of the head 2a or 2b through an angle of 2.06.degree.. The following area 43 is a PCM area having a length c which, for example, is equivalent to the rotation of the head 2a or 2b through an angle of 26.32.degree., and such area 43 is adapted for the recording therein, and reproducing therefrom of a PCM audio signal. The PCM area 43 is followed by a postamble area 44 which provides a back margin during after-recording, and which has a length d equivalent to a head rotation angle of, for example, 2.06.degree.. The area 45 is a guard area interposed between the track portion 31S and the track portion 31V, and has a length e which, for example, is equivalent to a head rotation angle of 2.62.degree..
The PCM audio data recorded in the area 43 of each track represent stereophonic audio signals for one field period which have been time base compressed and pulse code modulated, and which include an error correction code, an ID code and the like. The signals to be recorded in the areas 42, 43 and 44 are converted into bi-phase mark signals which have a frequency of 2.9 MHz to represent the value "0" and the frequency 5.8 MHz to represent the value "1".
The track portion 31V is shown to be divided into a major area 46 having a length equivalent to the rotation of the head 2a or 2b through an angle of 180.degree., and which extends from the guard area 45 to a scan end area 47 having a length equivalent to a head rotation angle of 5.degree., and at which the head moves out of contact with the tape 1. The area 46 has recorded therein one field of a frequency multiplexed video signal conventionally comprised of an FM signal which has been modulated by a luminance signal, a carrier chrominance signal which has been low band converted, an FM signal which has been modulated by a monaural audio signal, and a pilot signal used for tracking and which is provided with four different frequencies.
Since the ID code is recorded in the area 43 together with the PCM audio signal, such ID code may represent various kinds of information or data, such as, the month, day and year of recording, a program number or the absolute position of the respective track along the tape. The ID code is convenient for use in editing, or conducting a search when reproducing the recorded information.
Conventionally, the ID code is distributed and recorded together with the PCM data in the area 43 by an interleaving operation. Therefore, in order to correctly reproduce the ID code, it is necessary for the rotary head 2a or 2b to accurately scan the area 43 along a substantial portion of its length c. However, in the search mode, since the tape 1 runs at a speed which is, for example, 30 times the tape speed used in the normal recording mode, the rotary head 2a or 2b scans obliquely across the PCM area 43. Consequently, in the search mode, the ID code cannot be correctly reproduced in a reliable manner. In order to overcome the foregoing problem, consideration has been given to the recording, in the postamble area 44 of the track portion 31S, of an ID code in the form of sub-digital data, frequently called an index or subcode signal.
In the case where an NTSC video signal is being recorded, one field of such video signal recorded in the area 46 has a duration of 262.5 H (in which H is a horizontal period). Thus, the head 2a or 2b moves through the rotational angle of 180.degree. in the period 262.5 H, and the time for movement of the head through the postamble area 44 having a length d equivalent to a rotational angle of 2.06.degree. is 262.5 H.times.2.06/180, or approximately 3.0 H. Postamble signals consisting of all "1"s for the PCM audio data in the area 43 are recorded in the initial half of the area 44 which is scanned in the period 1.5 H, and the sub-digital data or subcode signal is recorded in the latter half of the area 44 which is also scanned in the period 1.5 H.
The subcode signal recorded in the latter half of the area 44 has a predetermined format and is provided with a code which is the same as, or similar to the ID code recorded with the audio signal in the PCM area 43. The subcode signal may include information, such as, the absolute position of the respective track on the magnetic tape 1, the cut number of the respective recorded contents, the month, day and year and the hour, minute and second when the recorded performance occurred. In the event that the subcode signal is to be erased or is unnecessary, an erasing code is recorded in the latter half of the area 44 and has a predetermined bit pattern, such as, alternately arranged "0"s and "1"s. In a manner similar to the signal recorded in the PCM area 43 of the track portion 31S, the subcode signal is recorded in the latter half of the area 44 after having been converted into a bi-phase mark signal having the frequencies 2.9 MHz and 5.8 MHz for representing the values "0" and "1", respectively.
When the subcode signal is recorded in the latter half of the postamble area 44 as described above, the subcode signal can be reliably reproduced even in a variable speed reproducing mode, such as, a search mode in which the tape is advanced at 30 times its normal speed. Further, after the audio and video signals have been recorded, the subcode signal alone can be independently recorded. Thus, for example, after completion of the editing of the tape, a subcode signal can be recorded for identifying the chapter number or the like. Similarly, after completion of the editing, if the subcode signal is no longer necessary, it can be erased by an erasing code.
When the subcode signal is being recorded, a switching pulse Ps (FIG. 2A) which is suitably synchronized with the rotational phase of the rotary head 2a or 2b then scanning the tape 1 is used as a reference. More specifically, a pulse signal P.sub.1 (FIG. 2B) which identifies the PCM area 43 and a pulse signal P.sub.2 (FIG. 2C) which identifies the postamble area 44 are suitably generated in predetermined timed relationships to the edge or switching position of the switching pulse Ps which is at a high level for one field period Ta and is inverted to a low level for the next field period Tb.
In the case where the area for the recording of the subcode signal is determined on the basis of the conventional switching pulse Ps as a reference, the recording position of the subcode signal will fluctuate in accordance with corresponding allowable fluctuations in the phase of the switching pulse Ps. Such fluctuations in the phase of the switching pulse Ps are practically unavoidable due to a jitter component in the drum servo, acceptable adjustment errors, temperature fluctuations, less than precise compatibility, that is, variations between mass produced VTRs and the like. In the event of repeated after-recording of subcode signals, fluctuations between the positions where the subcode signals are recorded may cause a previously recorded subcode signal and a newly recorded subcode signal to exist simultaneously on the magnetic tape 1.
For example, if an area 48a where a subcode signal is recorded is assumed to be shown in a reference position on FIG. 3A, it will be apparent that, when recording is effected in a VTR in which the switching pulse Ps deviates in one direction and thereby advances the position at which the recording of the subcode signal is initiated, the subcode signal is then recorded in an area 48b positioned as shown on FIG. 3B. On the other hand, if recording is effected in a VTR in which the switching position of the pulse Ps is deviated in the other direction, the area 48c in which the subcode signal is recorded may be positioned as shown on FIG. 3C. Therefore, if the magnetic tape has initially had the subcode signal recorded thereon in the area 48b shown on FIG. 3B, and then after-recording of the subcode signal is effected in a different VTR having characteristics represented by FIG. 3C, the subcode signal previously recorded in the area 48b and the subcode signal after recorded in the area 48c may simultaneously exist in the same track, as shown in FIG. 3D. In such case, erroneous operation may result from the fetching of the subcode signal recorded in the area 48b rather than the subcode signal after-recorded in the area 48c.
Heretofore, it has been proposed to precede the after-recording of the subcode signal by an erasing operation in which an erasing code is recorded over an erasing area that is substantially wider than the area in which the subcode signal is recorded so as to avoid the simultaneous presence on the tape of previously and after-recorded subcode signals, as shown in FIG. 3D. However, if the erasing area is made sufficiently wide to ensure that a previously recorded subcode signal will be entirely deleted, there is the danger that the PCM signal in the area 43 or the video signal in the area 46 will be damaged by the erasing code recorded in the relatively wide erasing area. In order to avoid such possibility of damage to the PCM signal or the video signal, it would be necessary to relatively severely limit the possible fluctuation of the switching pulse Ps to, for example, about 0.5 H. However, according to the existing standards for the 8 mm VTR, the allowable fluctuation of the switching position is .+-.1.5 H for the NTSC system.